1. Field of the Invention
The present invention generally relates to the non-destructive measurement of high aspect trenches in semiconductor materials, and more particularly to a non-destructive trench depth measurement method and apparatus.
2. Description of the Related Art
High aspect ratio holes or trenches formed in silicon (or other semiconductor material) typically must be measured for various reasons relating to the proper operation of the devices being made and the control of the manufacturing process. These trenches are typically on the order of microns deep. In the context of the present disclosure, the trenches are typically of a depth ranging from 0.25 to 300 microns. In addition, the trenches are usually of high aspect (depth/width) and in at least one dimension have a width parameter that is small (e.g., a micron or fraction thereof), thereby making optical or atomic force microscope measurement (AFM) difficult or impossible. As discussed below, the present invention can certainly measure trenches outside the above parameters, but this is the area where the greatest advantage is to be found.
Thus, as a practical example, the depth parameter of high aspect ratio trench structures characteristic of dynamic random access memory (DRAM) and embedded DRAM (EDRAM) memory devices are difficult or impossible to measure using conventional non-destructive techniques including top-down scanning electron microscope (SEM) and atomic force microscopy (AFM) and optical microscopy due to narrow width (0.25 micron or less) and large depth (8 microns or so).
To monitor the performance of the processes that create these trench structures, it is necessary to employ costly destructive methods such as cleavage SEM in which the wafer (or a part thereof) is actually destroyed, thereby decreasing yield and raising costs.
Thus, prior to the present invention, there has been no technique which solves the above-mentioned and other problems, by enabling the non-destructive depth measurement of trench structures using a combination of scanning electron microscopy and energy dispersive X-ray spectroscopy.
Additionally, existing methods suffer from other difficulties. For example, atomic force microscopes (AFM) employ tapered tips which cannot physically reach the bottom of high aspect ratio trenches. Even in instances where nanotube tips are employed, the bending of the tip and sidewall stiction prevent accurate measurement of extremely high aspect structures. Further, optical microscopy cannot penetrate light deep into opaque structures with openings smaller than about 0.5 microns, and even then with very limited results. SEM measurement is similarly constrained in that there is no clear line of sight means of escape for back-scattered or secondary electrons at the bottom of the trench to reach the detector, thereby making imaging of this region difficult, if not impossible.